In the past there has been utilized plug gages which, for example, have three point measurement contacts to centralize the bore gage within the hole and transmit the motion of the movable members back to an indicator such as a dial gage. It has also been suggested as, for example, in the Abramson U.S. Pat. No. 2,799,092 to provide opposed measuring contacts in the form of balls which engage grooves at the end of the measuring plunger or rod. Other types of balls that may be used with bore gages are disclosed, for example, in Lendi et al U.S. Pat. No. 4,058,901.
Instruments of this type have been found to have a small drawback in that the balls bear against the surface of the member that transmits the motion in a point contact form. This means that considerable wear can occur between the balls and the measuring rod which gives a high loading factor. For example, in torture tests the ball contact actually becomes distorted due to a peening action and considerable wear can be detected.